FIG. 1 illustrates the basic network architecture of a conventional radio access network 100, in form of a wireless local area network, WLAN, comprising a first access point AP1 110, a second access point AP2 120, and an Access Router, AR 150. The WLAN 100 may be connected to other network/s 160 such as e.g. the Internet and/or a UTRAN (Universal Terrestrial Radio Access Network). In FIG. 1, AP1 110 and AP2 120 are connected with AR 150 via Multicast-enabled Layer 2 Switches, M-L2S1 130 and M-L2S2 135, respectively, but many other possibilities exist. The APs may e.g. be connected directly to AR 150 without any intermediate M-L2Ss, or they may be connected to the same M-L2S which in turn is connected to AR 150. Since the Ethernet (IEEE 802.3) protocol is used for most of the WLAN access points today as layer 2 protocol to communicate with fixed network infrastructure, an M-L2S is identical with an Ethernet switch. A User Terminal, UT, 140 having a communication session, e.g. a data session or a Voice over IP session with a peer connected to the network 160 (e.g. Internet or UTRAN), routed via AP1, 110, normally performs a handover of the session from AP1, 110, to AP2, 120, whenever a certain handover criterion is fulfilled. The criterion is normally a function of the offered radio link quality and/or QoS (quality of service) of AP11 10 and AP2 120, respectively, so that the UT's 140 communication session will be routed through the access point offering the highest/best radio link quality/QoS, in a conventional manner. However, the handover criterion may be based on other aspects e.g. regarding accounting, security etc.
General problems regarding effective handover schemes for radio access networks relate e.g. to data loss minimization, interference suppression, packet delay minimization and to minimize network signaling.
More specifically, for the WLAN in FIG. 1 exploiting the IEEE 802 standard, the establishment of a security association between the UT 140 and a target AP, i.e. AP2 120 in case of a handover from AP1 110 to AP2 120 in FIG. 1, when applying a standard EAP (Extensible Authentication Protocol) authentication in accordance with the IEEE 802.11i security specification may pose a crucial issue regarding the caused interruption time, i.e. it may cause unacceptable packet delay/loss for real time applications such as voice and/or video. Such an authentication is carried out after that the UT 140 has been successfully associated with the new AP (AP2 120 according to above example), and the radio link connectivity between UT 140 and the old AP (AP1, 110) is thus already aborted. During the period between the initiation of association with a new target AP (AP2, 120) and completion of the EAP authentication and installation of the security parameters at the new target AP, no data (e.g. IP-) packets can be exchanged between UT 140 and AR 150 over the WLAN transmission path, which of-course constitutes a problem.
Another problem associated with the handover of TCP-sessions in radio access networks is that received packets being out of order will be retransmitted, which increases the radio interference, and the transmission rate may also be tuned down as a consequence, since the TCP interprets packets being out of order as a network congestion state.